Since the past, nitrile rubber (acrylonitrile-butadiene copolymer rubber) has been used as the material for rubber parts such as hoses, belts, and tubes for automobiles by taking advantage of its oil resistance, mechanical properties, chemical resistance, etc. Further, highly saturated nitrile rubber obtained by saturation by hydrogenating the carbon-carbon double bonds in the polymer main chain of the nitrile rubber is further excellent in heat resistance, so is being used for rubber parts such as seals, belts, hoses, and diaphragms.
On the other hand, the technique of mixing staple fiber into rubber such as highly saturated nitrile rubber so as to improve the mechanical properties of the obtained cross-linked rubber is known. However, it is difficult to make staple fiber uniformly disperse in rubber. If just adding staple fiber into rubber, there is the problem that the staple fiber entangled with itself and ended up forming clumps and it is impossible to obtain the desired properties.
In regard to this, Patent Document 1 proposes mixing staple fiber and a plasticizer into a copolymer rubber containing α,β-ethylenically unsaturated nitrile monomer units to obtain a master batch and thereby mixing the staple fiber into the highly saturated nitrile rubber in the state of the master batch. According to the art of this Patent Document 1, it is possible to make the staple fiber disperse well into the highly saturated nitrile rubber, but a plasticizer is used when obtaining the master batch, so as a result, the plasticizer in the obtained cross-linked rubber ends up becoming relatively great in amount, so sometimes bleedout of the plasticizer occurs. Depending on the application for which the obtained cross-linked rubber is used, bleedout of the plasticizer sometimes becomes a problem. Further, the cross-linked rubber obtained in Patent Document 1 is not sufficient in tearing strength either. Further improvement of the tearing strength has been sought.